An explanation of the asteroid hypothesisLuis Alvarez hypothesized that the asteroid had blown millions of tons of dust into the atmosphere, blotting out the sun, stopping photosynthesis and plant growth and hence, causing the global collapse of food webs and resulting in a major extinction.

Average ozone levelsThe graph shows the average October ozone levels recorded by Farman's group at Halley Bay from 1957 through 1984.

Balanced reporting (1 of 2)The public should be able to get information on all sides of an issue but that doesn't mean that all sides of the issue deserve equal weight.

Behaving scientificallyScience is open to anyone and benefits tremendously from the expanding diversity of perspectives. However, science only works because the people involved with it behave "scientifically."

Building a scientific argument - cartoon 1Scientific arguments involve three components: the idea (a hypothesis or theory), the expectations generated by that idea (frequently called predictions), and the actual observations relevant to those expectations (the evidence).

Building the argument for ozone depletionResearchers needed to incorporate nitrogen dioxide into the hypothesis — but they weren't sure how it would affect the expectations generated by the hypothesis. Would we still expect to see significant ozone depletion?

Building the argument for rapid ozone depletionRowland, Solomon, and Garcia's work suggested a modification to the original hypothesis: CFCs cause significant ozone depletion — and they do it much more rapidly with the help of polar clouds.

Chicxulub craterThe map shows the location of the Chicxulub impact crater, the site of a massive asteroid impact.

Comparing ozone and chlorine monoxide levelsA plot of chlorine monoxide and ozone concentrations from data collected by an aircraft passing through the Antarctic ozone hole. Outside the hole (left side of graph), ozone levels are high and chlorine monoxide levels are low, while the reverse is true inside the hole (right side of graph) — just as the Molina-Rowland hypothesis would lead us to expect.

Competing ideas (1 of 5)Scientists are more likely to put their trust in ideas that generate more specific expectations (i.e., are more testable).

Even theories change (1 of 4)Science is always a work in progress and even theories change. For example, in the 1600s classical mechanics was the accepted explanation of the movement of objects both in space and on Earth.

Even theories change (2 of 4)Then classical mechanics was one-upped by Albert Einstein's theory of special relativity because it explained more phenomena.

Even theories change (4 of 4)General relativity seems up for a change. For example, it doesn't mesh with what we know about the interactions between extremely tiny particles. Will physicists develop a new theory that simultaneously helps us understand the interactions between the very large and the very small?

Examining the source (1 of 2)When evaluating a media message about science, one of the first things to consider is where the information came from.

How the base pairs match upGiven the correct forms for the bases, Watson was able to figure out how adenine-thymine and guanine-cytosine pairs matched up, and formed weak hydrogen bonds with one another. Watson and Crick originally suggested that there were two bonds between guanine and cytosine but later it was found that a third existed.

Hypothesis of ozone depletionMolina and Rowland's over-arching hypothesis (that releasing CFCs into the atmosphere would cause significant ozone depletion) was based on many supporting hypotheses, backed by their own lines of evidence.

Magnetic flip-flopsAs new seafloor forms, the igneous rock records the current state of the Earth's magnetic field. Sedimentary rock layers forming at the bottom of the ocean may also record these magnetic flip-flops as sediment layers slowly build up over time.

Making assumptions (1 of 2)Much as we might like to avoid it, all scientific tests involve making assumptions — many of them justified.

Molecular structure of chlorine nitrateIn the presence of another molecule to serve as a catalyst (not shown), nitrogen dioxide (NO2) and chlorine monoxide (ClO), a byproduct of the breakup of ozone molecules by CFCs, react to form chlorine nitrate (ClONO2).

Multiple explanations for the extinction of dinosaursThe evidence supports the idea that a giant asteroid struck Earth at the end of the Cretaceous, but additional explanations (volcanic activity, global climate change, sea level change, and tectonic movements) may also have played a role.

Northern hemisphere ozone changeChange in the amount of ozone in the Northern Hemisphere over a 17-year period. Note the large decline in ozone during the winter months and how the winter decline is greatest at higher latitudes.

Ozone depletion - CFC phaseoutA graph showing world production of three major CFC types between 1970 and 1988. The dot marks the year (1977) that the US phaseout of CFCs was announced.

Ozone fluctuationOzone levels fluctuate so widely that it is difficult to detect subtle trends over a short-term period, as shown by these ozone measurements for the atmosphere over Switzerland taken between 1926 and 1975.

Ozone recoveryWith the CFC ban in place, atmospheric levels of chlorine are beginning to decline.

Peer reviewPeer review of a scientific study submitted for publication provides assurance that someone who knows what they're doing has double-checked it.

Products of deuterium fusionAccording to nuclear theory, deuterium atoms fuse and forming helium-4 which has a lot of energy — so much energy that it is unstable, so the atom discharges some of this energy by releasing a neutron, proton, or gamma ray.

Rutherford's argument (1 of 2)In the early 1900s, Ernest Rutherford and his colleagues performed this experiment to test the hypothesis that an atom's mass and positive charge are spread diffusely throughout the atom and found that their expectations and actual observations did not match at all.

Summing up scientific argumentsScientific arguments are formed by figuring out what we would expect to observe if a particular idea were true and then checking those expectations against what we actually observe. A match between expectations and observations lends support to the idea, while a mismatch helps refute the idea.

Testable ideasA scientific idea may require a lot of reasoning to work out an appropriate test, but to be scientific, an idea must be testable.

Testing an idea (1 of 3)Psychologist Edward Tolman wanted to know how rats successfully navigate their surroundings — for example, a maze containing a hidden reward. Would they build mental maps of the maze as they investigated it or would they learn to navigate the maze through stimulus-response?

Testing an idea (2 of 3)Tolman and his colleagues trained rats in a maze which offered them many different tunnels to enter first. One of the tunnels twisted and turned but consistently led to the reward, and the rats quickly learned to go down that tunnel.

Testing an idea (3 of 3)When the entrance to the reward tunnel was blocked, most of the rats picked a tunnel that led in the direction of the food, supporting the idea that rats navigate using something like a mental map.

Testing scientific ideasScientific testing occurs in two logical steps: (1) if the idea is correct, what would we expect to see, and (2) does that expectation match what we actually observe? Ideas are supported when actual observations (i.e., results) match expected observations and are contradicted when they do not match.

Testing the endosymbiotic hypothesis Testing the hypothesis: If some organelles evolved via endosymbiosis, we would expect to see a close genetic relationship between the organelles and bacteria.

Testing the hypothesis of ozone depletion (3 of 3)Chlorine monoxide is one of the products of ozone destruction. Since there is no other known source of chlorine monoxide, finding this chemical in the upper atmosphere would strongly support the idea that chlorine is destroying ozone.

Testing the idea of cold fusion (3 of 6)Pons' neutron results didn't agree with his heat measurements, with Jones' neutron results, or with established nuclear theory, which suggested no fusion should be occurring at all!

Testing the supernova hypothesisTo further test the supernova hypothesis:If a supernova had occurred, it would have also released plutonium-244 as well as iridium. Original tests looked positive but replicating the analysis showed that there was no plutonium in the sample, contradicting the supernova hypothesis.

Testing the two hypotheses (4 of 4)A group of scientists showed that if you remove tubule organelles from a eukaryotic cell, they can grow back. This discovery argued against the idea that these organelles evolved via endosymbiosis.

The argument for plate tectonics (1 of 6)Powerful scientific ideas generate many different expectations, not just one. As an example, let's return to the idea that the continents as we know them today were once joined together into a supercontinent and have been moving apart ever since. This idea generates many different expectations; we would expect to find corresponding fossils on now distant continents.

The supernova hypothesisAn existing hypothesis proposed that a supernova at the end of the Cretaceous had caused the extinction of dinosaurs. Supernovas throw off heavy elements like iridium — so the hypothesis seemed to fit with the team's discovery of high iridium levels.

Thymine and guanineThe visiting American chemist, Jerry Donohue, provided a key piece of evidence when he revealed that the forms given for thymine and guanine in most textbooks were wrong. Note the changes, indicated by the glowing hydrogens.

Two hypotheses for the rate of depositionAlvarez posed two hypotheses: either the clay was deposited over a few years and he would find no iridium, or over a few thousand years and the iridium would definitely be present.

Untangling media messagesMedia representations of science and science-related policy are essential for quickly communicating scientific messages to the broad public; however, some important parts of the scientific message can easily get lost or garbled in translation.

Updating evolutionary theoryMargulis didn't overthrow any of the core ideas of evolution, but she did force some of them to move over and make room for endosymbiosis.

Watson and Crick's first model of DNAWatson and Crick's model erroneously placed the bases on the outside of the DNA molecule with the phosphates, bound by magnesium or calcium ions, inside.

X-ray diffractionX-ray diffraction was a tool developed in the first half of the 20th century to infer atomic structure.